The aim of the proposed study is to examine the impact of treatment with artemether-lumefantrine (AL) on alleles of pfcrt and pfmdr-1 in P. falciparum isolates in an area of marked seasonal transmission in eastern Sudan. Most studies of resistance markers measure marker prevalence by DNA amplification, but we will also investigate gene expression using quantitative amplification of mRNA encoding pfcrt and pfmdr-1. The impact of genotype and gene expression levels on treatment outcome, and on the emergence and density of peripheral gametocytes will be examined.